1. Field of the Invention
The present invention relates to an apparatus for cleaning the outer peripheral surface of a printing cylinder such as a blanket cylinder of an offset printing machine, a plate cylinder of a gravure printer and so forth. More particularly, the present invention is concerned with a cylinder cleaning apparatus suitable for preventing jamming of the cylinder with a cleaning cloth which may otherwise be caused in the event of a breakage of the cleaning cloth.
2. Description of the Prior Art
A typical known printing cylinder cleaning apparatus will be described hereinunder. Although the following description is concentrated specifically on a blanket cylinder of an offset printing machine, it is to be understood that the same printing cylinder cleaning apparatus can be used for cleaning other types of cylinders such as, for example, a plate cylinder of a gravure printer.
An offset printing machine usually has three cylinders, namely, a plate cylinder, a blanket cylinder and an impression cylinder. These cylinders are arranged such that their axes extend in parallel with one another and such that they can be brought into mutual contact. A printing plate made of, for example, zinc or aluminum is wound on the plate cylinder, while a sheet-like blanket such as of rubber is wound on the blanket cylinder. The printing plate has a grained surface and is provided with a water repellent layer carrying an image of characters or a picture. The surface of the printing plate other than portions having images of characters or picture is dampened as water is supplied to the surface of the printing plate by means of dampening device. An oily ink is applied to the surface of the printing plate by means of an inking device. The ink will attach to the portion of the plate surface carrying the image because this area is not dampened, but will be repelled by other portions of the plate surface due to the water content held by the grained surface. The ink thus held on the plate cylinder is transferred to the blanket cylinder and is further transferred to a printing paper which passes through the nip between the blanket cylinder and the impression cylinder.
After several hours of printing operation, the blanket surface is contaminated due to accumulation of residual ink. It is therefore necessary that the surface of the blanket is cleaned periodically at a predetermined period. The cleaning operation heretofore has been conducted manually, requiring labor and time.
In order to obviate this problem, an apparatus has been developed which is capable of cleaning the blanket cylinder.
This apparatus has a cleaning cloth supply roll and a cleaning cloth take-up roll arranged in a pair on a pair of side plates which are mounted on the frame of the printing mahine. A continuous cleaning cloth is wound at its both ends on these rolls and are suitable tensed between these rolls. The cleaning cloth take-up roll is driven by a driving device to rotate at a predetermined speed. A stay having a substantially T-shaped cross-section is provided to extend in the direction of axes of these rolls. Both ends of the stay are fixed to the adjacent side plates. The portion of the stay opposing to the blanket cylinder is made hollow, and the surface facing the blanket cylinder is hermetically lined with a pressure pad made of an elastic member. A plenum chamber which is defined by the end of the stay and the pressure pad is communicated with an external air compressor. As the air compressor operates, compressed air is supplied to the plenum chamber so that the pressure pad is expanded, whereby the cleaning cloth sliding on the outer surface of the pressure pad is pressed against the blanket cylinder so as to wipe off contaminants on the blanket wound on the blanket cylinder thereby cleaning the blanket surface.
Usually, the cleaning is conducted while the blanket cylinder is rotated continuously. Therefore, this known printing cylinder cleaning apparatus suffers from the following drawback. Namely, there is a risk for the cleaning cloth to be broken or torn for various reasons. There also is a fear that the cleaning cloth sticks onto the surface of the blanket cylinder due to tackiness of the ink. In such cases, the cleaning cloth is inconveniently caught by the rotating blanket cylinder to reach the plate cylinder and the impression cylinder resulting in a serious accident in the printing machine.
A known system for overcoming this problem employs a rotary encoder fixed to the shaft of the cleaning cloth supply roll so as to output the number of revolutions of the roll shaft thereby enabling to detect a tearing of the cleaning cloth through the detection of a change of revolutions of the cleaning cloth supply roll.
This system, however, is still disadvantageous in that there is a certain time lag or delay for the printing machine to be actually stopped after receipt of the signal indicative of occurrence of the tearing of the cleaning cloth. In consequence, the torn cleaning cloth is caught by the blanket cylinder to cause a serious accident. In addition, the construction of the printing machine is impractically complicated due to provision of various equipments such as the rotary encoder.